The present invention relates to a method of making fiber preforms for use in manufacturing annular parts of composite material.
A particular but non-exclusive field of application of the invention is manufacturing brake disks out of composite material, in particular brake disks out of carbon-carbon composite.
Annular parts of composite material, such as brake disks, are constituted by fiber reinforcement known as a "preform" which is densified by a matrix. For carbon-carbon composite brake disks, the preform is made of carbon fibers, or of fibers constituting a precursor for carbon that is transformed into carbon by heat treatment after the preform has been made. A particular carbon precursor that is available in fiber form is preoxidized polyacrylonitrile (PAN). The preform can be densified using a liquid--by being impregnated with a liquid precursor of carbon, e.g. a resin, and then by transforming the precursor by heat treatment--or by chemical vapor infiltration.
A well-known method of making fiber preforms for composite material parts consists in superposing and needling together layers or plies of two-dimensional fiber fabric. The fiber fabric may be a woven cloth, for example. The cloth may optionally be covered in a fiber web to provide fibers suitable for being pulled by the needles through the superposed plies; this applies in particular when the fabric is made of fibers that are difficult to needle without breaking the yarns constituted by the fibers, in particular carbon fibers. Such a method is described in particular in documents FR-A-2 584 106 and FR-A-2 584 107 respectively for making preforms that are plane and for making preforms that constitute bodies of revolution.
An annular preform for a brake disk can be cut out from a thick plate made up of layers that are superposed flat and needled together. The loss of material is nearly 50%, and for carbon fiber preforms that represents a very large cost because of the cost price of the fiber material.
To reduce this loss, it has been proposed to make the preform by superposing and needling together annular layers, each made up of an assembly of a plurality of sectors. The sectors are cut out from a two-dimensional fabric such as a woven cloth.
The loss of material is less than when whole rings are cut out, however it is not negligible. In addition, the method is quite difficult to implement and to automate, particularly because of the need to position the sectors correctly while offsetting them from one layer to the next in order to avoid superposing lines of separation between sectors.
It might be thought that annular preforms could be cut from tubes made by winding and simultaneously needling a strip of cloth on a mandrel, as described in above-mentioned document FR-A-2 584 107. The method is relatively easy to implement without loss of fiber material. However, in the application to brake disks, and unlike the embodiments described above, the plies of the preform are then disposed perpendicularly to the rubbing faces. The smaller shear strength parallel to the plies can constitute a major drawback in the vicinity of the notches that are formed in the outer or inner periphery of the disks for receiving the splines that serve to transmit or take up the forces exerted on the disks.
Another known technique for making fiber preforms for composite material annular parts consists in using a textile product in the form of a spiral or helical strip which is wound flat to make up superposed turns. The textile product may be a woven cloth comprising helical warp yarns and radial weft yarns.
As described in documents FR-A-2 490 687 and FR-A-2 643 656, the spiral or helical shape is given to the cloth by using at least one frustoconical lap roller for warp yarns paid out from individual spools mounted on a creel. In cloth made in this way, the spacing between the radial weft yarns increases across the width of the helical cloth between its inside diameter and its outside diameter. In order to conserve a uniform character in the cloth over its entire width, the two above-mentioned documents propose inserting additional weft yarns that extend over a fraction only of the width of the cloth from the outside diameter thereof. That solution gives rise to significant extra cost in the manufacture of the cloth and is a source of non-negligible defects.
Another technique described in document EP-A-0 528 336 consists in using a tubular braid. Because of its deformability, the braid, once flattened, can be wound helically to form superposed annular turns that are bonded together by needling. During manufacture of the tubular braid, unidirectional elements can be inserted parallel to the axis of the braid. By appropriately varying the size and/or the mutual spacing of the unidirectional elements between opposite edges of the flattened braid, it is possible to introduce compensation for the variation in the density per unit area of the braid between its two edges as caused by the deformation imparted to the braid while it is being wound as a flat helix. Nevertheless, inserting unidirectional elements makes the braid less deformable. In addition, while the braid is being wound into a helix, the length described by the unidirectional elements varies considerably between the inside circumference and the outside circumference of the annular preform. During manufacture of the braid, document EP-A-0 528 336 does indeed envisage exerting tension to a greater or lesser extent on the unidirectional elements depending on whether they are situated close to the inside circumference or to the outside circumference of the annular preform. However, not only can that obtain only very limited compensation of the different lengths described by the unidirectional elements, there is also no guarantee that the tension will be maintained after the braid has been manufactured, thus giving rise to possible shrinkage and crimping of the unidirectional elements. In addition, inserting unidirectional elements in the tubular braid, particularly if the elements need to be inserted with different tensions, complicates manufacture of the preform considerably.